Photocatalysts are substances that cause charge separation in their molecules by light irradiation and donate generated electrons or holes, or both electrons and holes to another substance, thereby oxidizing or reducing the other substance. That is, photocatalysts are substances that induce various types of oxidation-reduction reaction by light.
Recently, photocatalysts have attracted attention as a material used for environmental clean-up, antifouling, defogging, sterilization, hydrogen production by water splitting, or light-electric energy conversion devices, a known example of which is a Graetzel cell (Non-Patent Document 1). Titanium oxide is most widely used as such a photocatalyst because of its high photocatalytic activity and low production cost.
The reason that titanium oxide causes charge separation by light irradiation is that it has characteristics of an optical semiconductor. The band gap of titanium oxide is about 3.2 eV. Accordingly, normal titanium oxide is excited only by irradiation of light in the ultraviolet region having a wavelength of 380 nm or less, which corresponds to the band gap energy of titanium oxide, and can be driven as a photocatalyst.
Considering the practical application of photocatalysts, sunlight is exclusively used for the driving thereof. However, light in the ultraviolet region, which can be utilized by a known titanium oxide photocatalyst, accounts for only 3% to 5% of the sunlight spectrum provided on the ground, and thus, the driving of the photocatalyst with a high efficiency is limited.
Accordingly, if the wavelength of light that can be used by a titanium oxide photocatalyst can be shifted to a longer wavelength side, visible light, which is a main component of sunlight, can be used. In such a case, it is expected that the photocatalyst can be driven under sunlight with a high efficiency.
Recently, nitrogen-doped titanium oxide has been reported as a titanium oxide photocatalyst capable of being driven with visible light (Non-Patent Document 2 and Patent Documents 1 to 6). By substituting some of the oxygen atoms of titanium oxide with nitrogen atoms, the band gap of titanium oxide becomes narrow. Consequently, the titanium oxide is excited by not only ultraviolet light but also light in the visible range having longer wavelengths and causes charge separation to exhibit a photocatalytic activity.
The nitrogen-doped titanium oxide that has been reported to date is produced by heating normal titanium oxide in a nitrogen stream or an ammonia stream at a high temperature in the range of 500° C. to 800° C. for several hours. Such high-temperature and high-nitrogen concentration conditions are essential to introduce nitrogen into titanium oxide in an amount sufficient to change the band gap.
However, such a heat treatment process at a high temperature generally degrades the photocatalytic activity. Since a photocatalytic reaction is conducted on the surface of the catalyst, a high specific surface area is required in order to exhibit a high activity. However, such a long-term heat treatment process at a high temperature causes densification of the photocatalyst, thereby markedly drastically decreasing the specific surface area.
The photocatalytic activity of titanium oxide largely depends on the crystallinity thereof, and in general, it is believed that metastable anatase-type titanium oxide has the highest activity (Non-Patent Document 3). However, when the anatase-type titanium oxide undergoes a heat treatment process at a high temperature, it transforms to rutile-type titanium oxide, which is the most stable crystal form of titanium oxide.    Patent Document 1: PCT Publication No. WO01/010552    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-255554    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2002-361097    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2003-190809    Patent Document 5: Japanese Unexamined Patent Application Publication No. 2003-340288    Patent Document 6: Japanese Unexamined Patent Application Publication No. 2004-97868    Non-Patent Document 1: B. O'Regan and M. Graetzel, Nature 353 (24) 737 (1991)    Non-Patent Document 2: R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y Taga, Science 2001, Vol. 293, P. 269    Non-Patent Document 3: K. Kato, A. Tsuzuki, H. Taoda, Y. Torii, T. Kato, and Y Butsugani, J. Mater. Sci. 29 (1994) 5911